This invention relates to apparatus for pulping paper making stock at relatively high consistencies, and more particularly to paper pulping apparatus in which a vaned rotor is utilized to circulate the stock within a tub.
Paper pulping apparatus typically includes a tub having a bottom wall and a cylindrical side wall extending upwardly therefrom, a vaned rotor centrally mounted on the bottom wall for rotation within the tub, and a screen or valve means located in or near the bottom wall to provide means for removal of the pulped paper stock. The rotor vanes include leading faces which are shaped and sized to contact stock near the bottom wall of the tub and propel it radially outwardly from the rotor against the side wall, which directs the stock upwardly. The rising stock flows to the center of the tub and then downwardly toward the rotating vanes. Thus during a pulping operation, the stock is caused to flow in a generally circular pattern, and the reduction in size of the individual particles of paper stock is due largely if not exclusively to the high sheer forces between particles of stock created by this flow pattern.
Many pulping devices of this construction presently in use are limited in application to paper making stock having a relatively low consistency, typically in the range of 4-7% solids content. If the consistency of the stock is raised appreciably above about 7%, the density and viscosity of the stock is such that it does not easily flow downwardly between the spinning vanes of the rotor. The vanes will displace an initial quantity of pulp outwardly, but cavitation will then occur since the displaced pulp is not replaced by downwardly flowing pulp.
In order to perform a pulping process upon stock having a consistency in excess of about 10%, special rotors have been developed. For example, in the Blakely et al. U.S. patent application Ser. No. 407,371, filed Aug. 12, 1982 and commonly assigned, a high consistency pulping apparatus is disclosed having a rotor with a vertically oriented and axially aligned feeding screw. The rotor includes vanes in the form of circumferentially-extending members with a half-crescent shape which are disposed about the periphery of a disk-shaped plate. The feeding screw is mounted in the center of the plate and includes a cylindrically-shaped body having a base which is spaced from the rotor vanes, and a helical screw flight which extends along the body and terminates at a squared end above the disk-shaped plate supporting the vanes.
When rotated in a tub containing stock at 12-13% consistency, the feeding screw guides the stock downwardly to the region of the rotor vanes which then propel it outwardly against the walls of the tub. The higher consistency stock climbs the walls of the tub, and baffles are employed to direct the pulp downwardly toward the tub center.
The potential advantages of high consistency pulping devices have been well recognized. For example, since the density of the stock is considerably higher for pulp stock having a consistency of about 14% than for stock having a consistency of about 7%, the shear stresses created during a high consistency pulping operation are significantly greater, so that the time required to perform a pulping operation with high consistency stock may be the same or even less than the time required for pulping the same volume of low consistency stock in the same pulping tub.
However, there often exist inherent disadvantages with high consistency pulping devices of the type previously described. For example, since the helical screw flight of the feeding screw terminates above the base of the rotor and is spaced from the rotor vanes, there may exist a tendency for the high density stock to collect and form a wedge between the underside of the trailing portion of the screw flight and the portion of the rotor base immediately beneath it. Another disadvantage is that there does not exist means to guide the stock from the squared trailing portion of the screw flight to the working faces of the vanes, so that voids may be created surrounding the working faces of the vanes.
Accordingly, there is a need for a high consistency pulping apparatus having a rotor which minimizes the likelihood of cavitation. There is also a need for a high consistency pulping apparatus in which the rotor provides means for guiding the high density stock in a smooth and uninterrupted path from a location adjacent the center of the rotor to the working faces of the vanes without the stock becoming wedged between the screw flight and rotor vane.